A GLASS ACT: An imbiber raises a something-tini at the Tribeca Grand Hotel, where, as at the Ace Hotel and others, tips are automatically added to the check.Lorenzo Ciniglio

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Big tips are no longer a “thank you” for good service — they’re now mandatory at many of Manhattan’s poshest bars.

At the trendy Gansevoort Hotel in the Meatpacking District, bartenders slap a compulsory 20 percent tip on all beer, wine and cocktails ordered at the bar.

That adds $7 to each $35 glass of Veuve Cliquot champagne.

At the Tribeca Grand Hotel, where a double Beefeater gin on the rocks costs a whopping $26, the bartender also includes an extra $4.68 tip — that’s an obligatory 18 percent gratuity — on your tab.

“It’s a hotel policy, because we have a lot of European tourists who don’t tip,” a bartender said.

The Standard Hotel on the West Side and the Lobby Bar at the Ace Hotel in Midtown also charge revelers a mandatory 18 percent gratuity on drinks, rather than leave the tip to the patron’s discretion. Both cite foreign tourists who don’t know it’s customary to tip your bartender as motive for the policy.

“New York City is now a tourist-based economy,” said nightlife impresario Steve Lewis. “The French and the Italians and the Spanish don’t tip, so it’s becoming a necessity at any place that’s dealing with a Euro crowd and wants to maintain its staff.”

But barflies accustomed to tipping their bartenders $1 a pour said they’re outraged by the sky-high mandatory tips.

“I was really surprised when the bill came,” said Kelli Avila, 25, who rang up a tab of about $70 on champagne and cocktails at the Ace Hotel Lobby Bar. “It’s pretty presumptuous.”

Breanne Schaap, 25, who was visiting the Ace Hotel, said the policy “only bugs me if I have bad service. Then I get really, really upset.”

The State Liquor Authority has no rules against mandatory tipping, according to a spokesman. But cash-strapped revelers who don’t want to pay up for bad service are not obligated to, said Warren County District Attorney Kate Hogan, who is well-versed in New York case law on the matter.

“Case law said a gratuity is within the discretion of the customers,” said Hogan, who dropped charges against a man who refused to pay a “required” 18 percent gratuity on a group dinner at Soprano’s Italian and American Grill in Lake George, NY. “The discretion to refuse payment is an essential element of a tip or gratuity,” Hogan told the Post about the 2004 incident.

The Post put this to the test yesterday, having a reporter order a $12 cocktail with vodka, lemon hibiscus syrup and Prosecco at the Ace Hotel bar. The check included a red stamp stating that an 18 percent tip was required, and when the reporter demanded that she pay whatever gratuity she wished, the bartended scratched it off without a fuss.